1. Field of the Invention
The present invention relates to a field-effect semiconductor device which operates at high speed. More particularly, it relates to a field-effect semiconductor device which controls carriers generated in the vicinity of a semiconductor heterojunction.
2. Description of the Prior Art
For enhancing the performance of a field-effect semiconductor device (hereinbelow, written "FET"), it is effective to first employ a semiconductor material exhibiting a high electron mobility and to secondly shorten a channel length.
In the first place, the employment of a semiconductor material having a high electron mobility will be described.
Gallium arsenide (GaAs) has an electron mobility much higher than that of silicon, and is a material suited for fabricating a high speed device. It has been revealed that, when a gate insulator layer in a MOS (Metal-Oxide-Semiconductor) type field effect transistor is replaced with an aluminum gallium arsenide (AlGaAs) crystal which is doped with a donor impurity, carriers are induced at the interface of the latter, so a field effect transistor can be fabricated. FIG. 1 is a band structure diagram of the active regions of this transistor. Numeral 13 designates an electrode portion, numeral 12 an AlGaAs layer which contains an impurity, and numeral 11 a GaAs layer which contains substantially no impurity. In addition, symbol F.sub.E denotes the Fermi level. In FIG. 1, numeral 15 indicates the carriers which are confined in a two-dimensional potential. The carriers 15 are supplied from a donor impurity level (14) in the AlGaAs (12) and travel within the GaAs containing no impurity, so that they are isolated in place from donor impurity ions. As a result, scattering due to impurity potentials decreases conspicuously, and a high mobility can be realized. However, in the case of fabricating the transistor by the use of electrons of high mobility, the transconductance is lowered by the addition of multiple donors in the AlGaAs, although a gate voltage is effective for the interface region. For preventing this drawback, it is desirable to use AlGaAs doped with no impurity as in the MOS structure. In case of a Schottky type gate, however, gaps often appear between a channel and source and drain electrodes, unlike the case of the MOS structure. In such case, the carriers are not induced in the gap portions when no donor impurity is added in AlGaAs or GaAs. Accordingly, the channel and the source and drain electrodes cannot be connected, and difficulty is involved in the operation of the transistor.
Examples of high-speed field effect transistors thus far described are reported in the following literatures:
(1) T. Mimura et al, Jpn. J. Appl. Phys. 19 (1980) L 225 PA0 (2) H. L. Stormer et al, Appl. Phys. Lett. 38 (1981) 691 PA0 (3) T. Mimura, Surf. Sci. 113 (1982) 454.